Formation of an alignment mark in the surface of an aluminum layer

ABSTRACT

An alignment mark appears on the surface of an aluminum wiring layer, when the wiring layer is formed to fill a recess. The recess is formed in oxide layers formed over the surface of a silicon substrate by etching. The depth of the recess is controlled to prevent direct contact between the wiring layer and metallic silicon of the silicon substrate. The wiring layer is thus prevented from chemically reacting with the metallic silicon so that deterioration of the oxide films and destruction of the alignment mark appearing on the surface of the wiring layer are prevented, even if the wiring layer is formed by sputtering aluminum on oxide layers and the recess at a high temperature.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a continuation application of application Ser. No. 09/228,576,filed Jan. 12, 1999, now U.S. Pat. No. 6,100,157 issued Aug. 8, 2000,which is a divisional application of Ser. No. 09/100,876 filed Jun. 22,1998, which is now U.S. Pat. No. 6,037,670issued Mar. 14, 2000, both ofwhich are hereby incorporated by reference in their entirety for allpurposes.

BACKGROUND OF THE INVENTION

This invention generally relates to formation of an alignment mark, andmore particularly relates to a method for forming an alignment markwhich is employed in the process of manufacturing a siliconsemiconductor device, especially in the process of forming a microscopicelectronic structure on the surface of a semiconductor or siliconsubstrate. The invention also relates to a method for forming astructure covering the alignment mark for protection thereof.

In the process of manufacturing a semiconductor device, thephotolithographic process is generally employed to process a desiredmicroscopic semiconductor structure on a semiconductor or siliconsubstrate surface. This photolithographic process typically consists ofthe steps of forming a photoresist film to cover the silicon substratesurface; projecting the light on the photoresist film to focus apredetermined light pattern thereon with help of an optical projector;developing in part the photoresist film exposed to the light; removingthe remaining photoresist film unexposed to the light; and etching thesemiconductor substrate surface portion as revealed by removing thephotoresist film until the desired pattern protected by the photoresistis appreciably obtained. This photolithographic process is repetitivelyperformed using several different patterns until the desired microscopicsemiconductor structure is completed on the semiconductor substratesurface. Therefore, in the photolithographic process, it is strictlyrequired to accurately align the pattern previously formed on thesubstrate with a pattern which is to be formed next thereon. In otherwords, the photolithographic process requires a so-called precisealignment process for aligning one pattern with the next. In order toaccurately carry out this process, some marks are employed as alignmentmarks which are provided in predetermined positions on the substratesurface, and relative positioning between patterns is performedreferring to these marks.

Typically, formation of these marks may be simultaneously executed inparallel with other processes such as a process for forming anelectrical wiring over the substrate. This process will now be describedwith reference to FIGS. 4 and 5 showing a prior art method in whichformation of the alignment mark is carried out in parallel withformation of an aluminum wiring over the substrate. At first, a siliconoxide film layer 12 is formed to cover the surface of a siliconsubstrate 10. While some holes for use in the aluminum wiring are boredby etching the oxide film layer 12 with a suitable etchant, formation ofrecesses 14 is carried out by etching the oxide film 12 at predeterminedpositions where alignment marks are expected to be located. As shown inFIG. 5, after the above etching process of the oxide film layer 12, aconductive material such as aluminum (Al) is sputtered over the oxidefilm layer 12 to form an Al wiring layer 16. When the Al wiring layer 16is completed, such a recess as indicated in the figure comes out on thesurface of the Al wiring layer 16, in correspondence with the positionof each recess 14 formed by etching the oxide film layer 12 in part.Thus, these recesses can be used as alignment marks AM. As describedabove, the recess 14 plays a role as a base which defines the positionof a designed or planned alignment mark AM. Therefore, for the purposeof simplifying the description, the recess 14 will be referred to as‘base recess’ hereinafter in this specification as well as in therecitation of claims as attached hereto.

In case of forming the base recess 14 for alignment mark AM by etchingthe oxide film layer 12 in part, its depth has to be carefullycontrolled, as shown in FIGS. 4 and 5, to avoid that the base recess 14reaches the silicon substrate 10 by excess etching. Despite the controlof this etching process, however, such excess etching might sometimeshappen depending on unexpected change in the etching condition, forinstance etching time, etching temperature, etchant, etc. Once suchexcess etching has happened, the base recess reaches the siliconsubstrate, so that the Al wiring layer 16 directly comes in contact withmetallic silicon of the silicon substrate 10 when the Al wiring layer 16is formed over the oxide film layer 12. Aluminum and silicon areessentially so active to each other that an Al—Si alloy is formed at theinterface therebetween, if the Al wiring layer 16 is formed at a hightemperature such as 500° C. Therefore, according to the prior art methodas described above, when aluminum wiring layer 16 and silicon substrateSi come in contact with each other under such high temperaturecondition, Al—Si chemical reaction rapidly and vigorously proceeds toproduce a reaction layer 18, and the oxide film layer 12 is violentlyattacked and destroyed during this chemical reaction, as shown in FIG.6, thereby the desired alignment mark AM being eliminated.

OBJECT AND SUMMARY OF THE INVENTION

Accordingly, the invention has been made in view of such a problem asdescribed above. Therefore, the first object of the invention is toprovide a novel and improved method for forming alignment mark which canbe stably maintained without being destroyed by rapid and vigorous Al—Sichemical reaction even when the alignment mark and the Al wiring layerare simultaneously formed at a high temperature.

Another object of the invention is to provide a novel and improvedmethod for covering an alignment mark, according to which the alignmentmark already formed on the silicon substrate can be protected from beingdestroyed by rapid and vigorous Al—Si chemical reaction.

In order to solve such problem as mentioned in the above, according tothe first aspect of the invention, there is provided a method forforming an alignment mark, which includes the steps of forming an oxidefilm over the surface of a silicon substrate, forming a base recess byetching the oxide film in part, and forming an Al wiring layer over theoxide film so as to fill up the base recess therewith, thereby causing amark as an alignment mark to come out at a point on the surface of theAl wiring layer in correspondence with the position of the base recess.In this case, according to the present invention, the base recess isformed such that it has a depth less than that (thickness) of the oxidefilm, so that the metallic silicon of the silicon substrate can not makeany direct contact with the Al wiring layer.

As described above, since the alignment mark according to the inventioncan be formed without causing any direct contact between the Al wiringlayer and the metallic silicon of the silicon substrate, there is nopossibility that they chemically reacts with each other. Thus, there iscaused neither deterioration in the quality of the oxide film, nordestruction of the alignment mark formed on the surface of the Al wiringlayer.

To put it more concretely, it may be possible to achieve such analignment mark as described in the above by using a method including thesteps of additionally forming the second oxide film over the oxide filmwhich is initially formed over the silicon substrate surface, and thenforming the base recess by etching this second oxide film in part. Withthe addition of the second oxide film, there will be further reduced apossibility that the metallic silicon of the silicon substrate isexposed to the inside of the base recess, even though the depth of thebase recess is made deeper to some extent, thus the Al wiring layerbeing prevented from making a direct contact with the metallic siliconof the silicon substrate.

Furthermore, according to the second aspect of the invention, analignment mark may be obtained by using a method including the steps offorming an aluminum layer over the oxide film which is initially formedover the silicon substrate surface, then forming second oxide film overthe aluminum layer, forming the base recess by etching the second oxidelayer in part, and finally forming the Al wiring layer over the secondoxide film. With addition of this aluminum layer, there is eliminated apossibility that the base recess for alignment mark reaches the metallicsilicon of the silicon substrate, penetrating through the aluminumlayer. Consequently, the metallic silicon of the silicon substrate isnot exposed to the inside of the base recess and the Al wiring layer isprevented from making a direct contact with the metallic silicon of thesilicon substrate.

Still further, according to the third aspect of the invention, there isprovided a structure for covering the alignment mark, which is formed bya method including the steps of forming an oxide film over the surfaceof a silicon substrate, forming a base recess by etching the oxide filmin part, and forming the second oxide film over the initial oxide filmover the silicon substrate so as to fill up the base recess alreadyformed therein. According to this covering structure, since the baserecess formed in the oxide film over the silicon substrate is filled upwith the second oxide, the Al wiring layer to be formed over the secondoxide film can not come in contact with the metallic silicon of thesilicon substrate. Consequently, there is caused neither Al—Si chemicalreaction nor destruction of the base recess already formed in the oxidefilm over the silicon substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features of the invention and the concomitantadvantages will be better understood and appreciated by persons skilledin the field to which the invention pertains in view of the followingdescription given in conjunction with the accompanying drawings whichillustrate preferred embodiments. In the drawings:

FIG. 1 is a longitudinal sectional view showing the structure of analignment mark according to the first embodiment of the invention;

FIG. 2 is a longitudinal sectional view showing the structure of analignment mark according to the second embodiment of the invention;

FIG. 3 is a longitudinal sectional view of a silicon substrate for thepurpose of explaining a method for covering a base recess according tothe third embodiment of the invention;

FIG. 4 is a diagram for explaining the prior art process of forming analignment mark;

FIG. 5 is a diagram for explaining the prior art process of forming analignment mark; and

FIG. 6 is a diagram showing a state of a destroyed alignment caused byAl—Si chemical reaction.

PREFERRED EMBODIMENT OF THE INVENTION

Preferred embodiments according to the invention will now be describedin the following with reference to accompanying drawings.

FIG. 1 is a longitudinal sectional view showing the structure of analignment mark AM according to the first embodiment of the invention.This alignment mark AM is applied to the case where an Al wiring isformed over a field oxide film.

As shown in the figure, the first oxide film layer 104 is formed tocover the surface of a silicon substrate 102. This oxide film layer 104is formed by oxidizing the surface of a cleaned silicon substrate 102through a known ordinary oxidation treatment such as thermal oxidationwith the help of an oxidation furnace, natural oxidation by leaving thesilicon substrate in the high level clean room air for a predeterminedperiod of time, or the like.

The second oxide film layer 106 is formed over the first oxide filmlayer 104. This second oxide film layer 106 consists of two oxide filmlayers 106 a and 106 b which are stacked up one by one on the film 104.Formation of these oxide film layers 106 a and 106 b may be achieved byusing a known method as properly selected, for instance the method ofchemical vapor deposition (CVD). Alternatively, this second oxide filmlayer 106 may be formed as a single oxide film layer or as a multipleoxide film layer including three or more oxide film layers.

As shown in the figure, the second oxide film layer 106 includes thebase recess 108, which may be formed by gas etching using a fluoride gasas an etchant such that it takes a preset position on the oxide filmlayer 106.

On the second oxide film layer 106, there is still further provided anAl wiring layer 110, which is formed by sputtering aluminum as a wiringmaterial on the oxide film layer 106. As a result of forming the Alwiring layer 110 over the second oxide film layer 106, the alignmentmark AM comes out on the surface of the Al wiring layer 110 taking aposition in correspondence with the base recess 108.

As described in the above, according to the embodiment shown in FIG. 1,the second oxide film layer 106 is additionally formed to cover thefirst oxide film layer 104 over the silicon substrate 102. Accordingly,when etching the base recess 108, even if the depth of the base recess108 is made deeper to some extent by some reason, the metallic siliconof the silicon substrate 102 is prevented from being exposed to theinside of the base recess 108. If there is a high possibility thatexcess etching of the base recess 108 might happen due to theunavoidable or uncontrollable change in the environmental condition, thesecond oxide film layer 106 may be made thicker by increasing the numberof its component film layers.

FIG. 2 is a longitudinal sectional view showing the structure of analignment mark AM according to the second embodiment of the invention.This alignment mark AM is applied to the case where a multiple wiringstructure is formed over the silicon substrate.

In this embodiment, the first oxide film layer 204 is formed over thesurface of the silicon substrate 202 by means of the thermal oxidationprocess or the natural oxidation in which the silicon substrate is keptin the high level dust-free clean room air for a preset period of time,or the like. Next, a lower Al layer 206 is formed over the first oxidefilm layer 204. This lower aluminum layer 206 is formed by sputteringaluminum on the first oxide film layer 204 in the same way of formingthe Al wiring layer 110 in the first embodiment as mentioned in theabove.

Furthermore, the second oxide film layer 208 is formed over the lower Allayer 206. Formation of this second oxide film layer 208 is carried outthrough the CVD process in the same way as formation of the second oxidefilm layer 106 according to the first embodiment. In this example shownin FIG. 2, the second oxide film layer 208 is formed as a multiple filmlayer including two or more film layers.

In the next, the base recess 210 for alignment mark AM is formed takinga predetermined position on the second oxide film layer 208, by gasetching using a fluoride gas as an etchant in the same way of formingthe base recess 108 in the above first embodiment.

An upper Al wiring layer 212 is then formed over the second oxide filmlayer 208. This upper Al wiring layer 212 may be formed by sputteringaluminum on the second oxide film layer 208 in the same way of formingthe Al wiring layer 110 according to the first embodiment. Formation ofthe upper Al wiring layer 212 over the second oxide film layer 208causes the alignment mark AM to come out on the surface of the upper Alwiring layer 212, taking a position corresponding to the base recess210.

According to the embodiment as shown in FIG. 2, the etchant such as afluoride gas used for etching the second oxide film layer 208 can notetch the lower aluminum layer 206, which rather functions as a barrieragainst such etchant. Therefore, when forming the base recess 210, thereis no possibility that the metallic silicon of the silicon substrate 202is exposed to the inside of the base recess 210.

In any event, when forming the base recesses 108 and 210 according tothe first and second embodiments of the invention, there is causedneither exposure of the metallic silicon of the silicon substrate 102 or202 to the inside of the base recess 108 or 210, nor direct contactbetween the metallic silicon of the silicon substrate and the Al wiringlayer 110 or the upper Al wiring layer 212. Consequently, there happensneither the Al—Si chemical reaction, nor deterioration in the quality ofthe first and second oxide film layers 104 and 204, nor destruction ofthe alignment mark AM. Therefore, the alignment mark AM can beaccurately formed and stably maintained, thereby enabling the precisepattern alignment to be achieved. As shown in FIG. 2, in case of formingthe lower Al layer 206 over the surface of silicon substrate 202, it ispreferable to form the oxide film layer 204 covering the surface of thesilicon substrate 202 in order to avoid the direct contact between thelower Al layer 206 and the metallic silicon of the silicon substrate202.

FIG. 3 is a longitudinal sectional view of a silicon substrate 302 forthe purpose of explaining a method for covering a base recess accordingto the third embodiment of the invention. As shown in the figure, thefirst oxide film layer 304 is formed over the surface of the siliconsubstrate 302 by means of thermal oxidation, natural oxidation, and soforth. In the next, a base recess 306 is formed in the first oxide filmlayer 304 by etching, and so forth. The silicon substrate 302 as shownhas a structure identical to the prior art structure which has beenalready described referring to FIG. 4.

In case of forming the base recess 306 by etching and so forth the firstoxide film layer 304 which is generated by oxidizing the surface of thesilicon substrate 302, since the first oxide film layer 304 isrelatively thin, if the depth of the base recess 306 is unexpectedlydeepened due to change in the process condition such as process time,temperature, etchant, and so on, there is a possibility that themetallic silicon of the silicon substrate 302 is exposed to the insideof the base recess 306. If the Al wiring layer is formed over the firstoxide film layer 304 leaving such a state of the base recess 306 as itis, the Al wiring layer directly comes in contact with the metallicsilicon of the silicon substrate 302, which would never fail to causethe problems as previously described referring to FIGS. 4 through 6.

Therefore, the third embodiment is characterized in that a second oxidefilm layer 308 is additionally formed over the first oxide film layer304 which is formed the base recess 306, thereby filling up the baserecess 306 formed in the first oxide film layer 304 with the secondoxide film 308. The second oxide film layer 308 may be formed by using aproper method such as the CVD method.

According to this method in the case of forming Al wiring layer 310,since the base recess 306 is already filled up with the second oxidefilm layer 308, it is avoided that the Al wiring layer 310 directlycomes in contact with the metallic silicon of the silicon substrate 302,so that no Al—Si chemical reaction is caused thus enabling the baserecess 306 in the first oxide film layer 304 to be protected fromdestruction.

As has been discussed in the above, according to the alignment mark asformed according to the invention, since the metallic silicon of thesilicon substrate can not come in contact with the Al wiring layer,there is caused neither Al—Si chemical reaction between silicon andaluminum, nor deterioration in the quality of the oxide film layer.Consequently, the alignment mark on the surface of the Al wiring layeris never damaged or destroyed.

According to the method of covering the base recess of the invention,since the base recess which is already formed in the first oxide filmlayer over the silicon substrate is concealed by the second oxide filmlayer, the Al wiring layer formed over the second oxide film layer cannot come in contact with the metallic silicon of the silicon substrate.Therefore, no Al—Si chemical reaction is caused, thus no destructionbeing caused over the base recess already formed in the first oxidefilm.

Preferred embodiments of the structure of the alignment mark and theconcealing structure of the same according to the invention have beendiscussed thus far with reference to the accompanying drawings. However,the invention should not be limited by these embodiments. It will beclearly understood by one skilled in the art that various changes andmodifications of the embodiments may be made within the technicalthoughts as recited in the scope of claim for patent as per attachedhereto and that those changes and modifications should also fall in thetechnical scope of the invention.

The entire disclosure of Japanese No. 9-187550 filed on Jun. 27, 1997including specification, claims, drawings and summary is incorporatedherein by reference in its entirety.

What is claimed is:
 1. A method for forming an alignment mark as used ina process of manufacturing a silicon semiconductor device, comprising:forming a first oxide film directly on a surface of a silicon substrate;forming a lower aluminum layer directly on said first oxide film;forming a second oxide film directly on said lower aluminum layer;forming a base recess by etching said second oxide film in part at apredetermined position thereon; and forming an upper aluminum layer oversaid second oxide film to fill up said base recess therewith, therebyforming a mark as an alignment mark on a surface of said upper aluminumlayer, at a position in correspondence with a position of a referencemark.
 2. A method as claimed in claim 1, wherein said aluminum layer isformed by sputtering aluminum on the oxide layer at a high temperature.3. A method as claimed in claim 1, wherein said base recess is etched byusing fluoride gas as an etchant.
 4. A method for forming an alignmentmark as used in a process of manufacturing a silicon semiconductordevice, comprising: forming a first oxide film over a surface of asilicon substrate; forming a lower aluminum layer over said first oxidefilm; forming a second oxide film over said lower aluminum layer;forming a base recess by etching said second oxide film in part at apredetermined position thereon; and forming an upper aluminum layer oversaid second oxide film to fill up said base recess therewith, therebyhaving a mark as an alignment mark come out on a surface of said upperaluminum layer, at a position in correspondence with a position of areference mark, wherein said second oxide film has a multiple oxidelayer structure.